import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class Solution15 {
    //446. 等差数列划分 II - 子序列
    public int numberOfArithmeticSlices(int[] nums) {
        Map<Long, List<Integer>> hash = new HashMap<>();
        int n = nums.length;
        int[][] dp = new int[n][n];
        for (int i = 0; i < n; i++) {
            long tmp = (long)nums[i];
            if (!hash.containsKey(tmp)) {
                hash.put(tmp,new ArrayList<>());
            }
            hash.get(tmp).add(i);
        }
        int sum = 0;
        for (int j = 2; j < n; j++) {
            for (int i = 1; i < j; i++) {
                long a = 2l * nums[i] - nums[j];
                if (hash.containsKey(a)) {
                    for (int k : hash.get(a)) {
                        if (k < i) {
                            dp[i][j] += dp[k][i]+1;
                        } else {
                            break;
                        }
                    }
                }
                sum += dp[i][j];
            }

        }
        return sum;
    }
}
